Many disposable sheet goods such as toilet tissue, paper towels, gift wrap, aluminum foil and the like, are sold in the form of a roll supported by a tubular paperboard core. Because of the strength required in the paperboard core during the process of winding the disposable sheet goods onto the core, the core is normally formed of at least two radial layers, which in turn, are formed from separate spirally wound paperboard plies. Each of the spirally wound paperboard plies forms a helical seam extending longitudinally along the paperboard tube resulting from abutment of the longitudinally extending edges of the ply along the length of the tube. During the tube manufacturing process, paperboard plies forming radially adjacent layers are wound onto a mandrel so that their respective seams are displaced longitudinally from each other. In other words, the helical seams of the adjacent layers do not overlap.
The paperboard tube making process is conducted by winding the inside paperboard layer onto a stationary mandrel while simultaneously winding one or more exterior paperboard ply over the exterior of the first plies. An adhesive coating is applied to the exterior face of the inside paperboard ply and/or to the interior face of the adjacent exterior paperboard ply. As a result, radially adjacent plies forming separate layers adhere strongly to each other so that the tube can have considerable strength. Although each of the spirally wound layers includes a continuous helical seam, the composite tube formed from several layers does not readily unravel because the seams in adjacent paperboard layers are offset longitudinally from each other as mentioned above, and because of the substantial surface bonding between adjacent layers.
Particularly in those cases where the paperboard tube is used as a core support for a disposable sheet material such as paper towels, toilet tissue, or the like, it is highly desirable to minimize the cost of the paperboard core. This has been achieved in typical commercial practice by minimizing the number of layers of paperboard used to form the core and by minimizing the cost associated with the paperboard forming each of the layers. Accordingly, commercially available cores are preferably formed from only two layers and each layer is formed a relatively inexpensive and weak paperboard, typically of relatively low density and having a high content of recycled material.
As will be apparent, there is a limit to the minimum strength of paperboards that can be used to manufacture paperboard cores. Thus, the cores cannot be made from materials which are so thin and/or weak that they will not form a self-supporting structure upon being wound into helical form because the tube structure must provide support to the sheet material which is wound onto the core. Similarly, the paperboard tube must be formed from at least one layer, and in commercial practice, at least two paperboard layers are used. Two layers are needed to provide the necessary strength to the cores which stems from the bonding and proper alignment of the multiple layers. Moreover, the single continuous helical seam running along the length of paperboard tubes formed of only a single layer are apt to spirally unravel along the seam.
Various attempts have been made to make paperboard tubes from a single layer of paperboard by forming an overlap joint along the helical seam. Thus, attempts have been made to overlap one edge of the ply onto the top of the other edge of the ply as the ply is wound onto the mandrel. However, these attempts have not resulted in production of a commercial paperboard core product when a relatively weak, low basis weight material is used because of various difficulties.
One problem associated with overlapped joints is the uneven exterior and interior surface which normally results. The tube is thicker in the overlapping joint area and thus includes a raised helical seam extending along the exterior of the tube surface. Moreover, a corresponding inside surface of the tube can also be uneven; in other words, the inside surface of such tubes can also include a helical raised region extending from end to end of the tube. These uneven inside surfaces can be problematic for inserting the tube onto a winding mandrel and/or removing the tube from the mandrel. Similarly, the exterior uneven surface can be problematic as it can impact negatively on the appearance on the material wound onto the tube.
In order to eliminate the raised regions associated with overlapped helical joints, paperboard plies having edge portions which are thinner than the middle portion of the ply have been used in an attempt to form an acceptable single ply tube having an overlapped edge seam of a thickness substantially the same as the non-overlapped portions of the tube wall. However, in practice, the costs associated with forming the thinner edges of the paperboard plies can substantially increase the cost of the ply. Thus, the process of forming the thin area on the edges of the paperboard ply must desirably result in an edge having an uniform thickness. Particularly when the paperboard is relatively inexpensive and thus, relatively weak, substantial efforts are required not to overly deform the edges, while at the same time deforming the edges sufficiently to achieve the desired degree of thickness reduction. In practice, edges of plies have been treated to decrease their thickness for the formation of paperboard tubes, by a grinding or compressing process in which the edges of the paperboard ply are ground with an abrasive wheel, or compressed between compressing rollers to decrease its thickness. However, as indicated above, costs associated with such treatments substantially increase the costs of the paperboard plies.
Although such compressed edge plies have been used by the assignee of the present application to form a single layer paperboard tube, the paperboard used in this process has been a relatively high strength, high basis weight paperboard, having a basis weight of about 75lbs/1000 sq ft., and a thickness of about 0.025 inch. These tubes have been used to support relatively expensive gift wrapping papers in which the importance of the appearance of the product justifies a higher cost. However, the costs associated with such paperboard is generally too high for use in the production of core support tubes for toilet paper and paper towels.
As indicated previously, compressing and/or grinding of the edges of paperboard made from relatively low basis weight, relatively low thickness paperboards can be difficult to accomplish with uniformity and can add substantial cost to the paperboard plies. In addition, it has been found in practice that an overlap joint is difficult to achieve in practice when relatively low basis weight, relatively weak paperboard plies are used. When an overlapped joint is formed, substantial pressure must be applied to the tube-forming ply during the spiral winding process. This is necessary so that the overlapping edge will make substantial and uniform contact along the length of the tube. At the same time, the primary portion of the paperboard layer must make substantial contact with the supporting mandrel. If either of these conditions are not met, the paperboard tube will have an uneven, wrinkled appearance and will not be uniformly bonded along the overlapping joint. However, with low basis weight, weak paperboard plies, the tension which must be applied to the plies during the winding process in order to achieve such contact can result in the tearing of the paperboard plies, which in turn, results in shutting down of the tube making process.
Paper of variable thickness has been proposed for uses in which little or no strength is required. For example, U.S. Pat. No. 701,734 to Jenks discusses a process for producing variable thickness paper that can be used as pages in bound books. The method includes compressing portions of a wet paper web with a roller so that the web includes a plurality of longitudinal channels that are thinner than the remaining portions of the web. However, the papers of Jenks would not have the strength of paperboard which is needed for structural uses such as for forming a tubular core for carrying sheet material. U.S. Pat. No. 768,422 to Case discusses a papermaking process which produces multi-ply paper with gradually thinned edges. The individual plies are produced by a series of adjacent and radially aligned cylinders, each of which produces an individual ply. The plies are combined in an overlying fashion to form a multi-ply paper. All of the cylinders have raised longitudinal edge portions that define the width of the ply. The first cylinder in the series produces the narrowest ply. Each successive cylinder produces a slightly wider ply than the preceding cylinder. As the plies are combined, the resulting product is a paper sheet having a thick central portion that thins gradually at its edges.